There are known bearing units for a vehicle wheel hub provided with special vents and sealing devices for blowing into the tyre air pressurized by a source of pressurized air mounted on board of the vehicle. These solutions allow to adjust and/or monitor the air pressure in the tyres.
For a better understanding of the state of the art and problems inherent thereto, there will be at first described a bearing unit of the above mentioned type, with reference to FIG. 1 of the accompanying drawings. A bearing unit of this kind is known, for example, from EP-713 021, EP-656 267, U.S. Pat. No. 5,503,480, DE-37 38 529, FR-2 714 943.
In FIG. 1, a bearing unit comprises an outer race 1, an inner race 2 formed by two axially adjacent half-races 2a, 2b and two sets of bearing balls 3, 4 interposed between the outer race 1 and the inner half-races 2a, 2b. In a radial plane located between the two sets of balls 3, 4 there are several outer radial ducts 5 passing through the bearing outer race 1, and several inner radial passages 6 obtained through the inner bearing race 2. The inner passages 6 are generally defined by the coupling of complementary recesses formed according to a specular symmetry on facing surfaces of the two inner half-races 2a, 2b. 
Mounted in the annular space defined by the outer race 1, the inner race 2 and the two sets of balls 3, 4 is a sealing device that allows pressurized air to pass through the outer 5 and inner 6 ducts of the bearing. The sealing device is constituted by two annular sealing members 7 facing one another axially and disposed symmetrically with respect to the radial plane in which the ducts 5 and 6 of the bearing unit lie. The sealing members define an intermediate annular chamber 8. Air pressurized by a pressurized air source mounted on board of the vehicle, which may be part of an automatic system or a system controlled by the driver, passes through specials ducts obtained in the suspension standard of the wheel where the bearing is housed, passes through the outer ducts 5, in the intermediate annular chamber 8, through the inner ducts 6, and from here is conveyed through other ducts to the wheel rim and finally the tyre.